1. Technical Field
The present invention relates to a laminated nonwoven fabric, and the invention relates to a laminated nonwoven fabric used for, for example, a filter medium of air purifier.
2. Description of the Related Art
Nonwoven fabrics as a filter medium to be used for air purifiers and the like are required to have as basic performances high long-term dust collection efficiency (dust-capturing capability) and low pressure loss (resistance when a fluid passes through the nonwoven fabric). However, these are reciprocal properties, so that it is difficult to make the both compatible with each other. For example, in order to improve the dust collection efficiency, a basis weight (density) of the nonwoven fabric has only to be increased. But, if the basis weight is increased, a flow channel of the fluid becomes small. Thus, the resistance to the fluid becomes large, so that the pressure loss becomes high. Furthermore, as the dust collection proceeds, whereby a void of the nonwoven fabric becomes small, though the dust collection efficiency is more enhanced, the pressure loss increases, too. Since the dust collection is achieved due to the matter that the dust-containing fluid passes through an interior of the nonwoven fabric, in order to allow the fluid to pass through an interior of the nonwoven fabric with high pressure loss, it is necessary to increase a motor output (sucking force) of an air purifier or the like. For that reason, there are encountered such problems that the electric power for use becomes large; and that the noise becomes large. In the case where an air purifier or the like is placed for household use in a room, attention to the noise is more important.
In order to enhance the dust collection efficiency, WO2008/130019 teaches that two kinds of fibers having a different average fiber diameter from each other are uniformly entangled.
In addition, in WO2013/121733, first nanofibers having a large average fiber diameter and second nanofibers having a smaller average fiber diameter than the first nanofibers are used and arranged on a base material such that a basis weight of the first nanofibers is smaller than a basic weight of the second nanofibers. According to this, it is taught that not only the dust collection efficiency is improved, but also an increase of the pressure loss is suppressed.
However, according to the method of WO2008/130019, the pressure loss is liable to become large. Even by the method of WO2013/121733, it may not be said that it is satisfactory to make both high dust collection efficiency and low pressure loss compatible with each other.